1. Field of the Invention
The present invention relates to sensors for detecting a substance in liquid using a surface acoustic wave element (SAW element), and more specifically, to a sensor for detecting a substance in liquid, the sensor including a sensing SAW element and a reference SAW element.
2. Description of the Related Art
Various sensors have been developed for detecting a substance (detection-target substance) in liquid, for example, a protein. For example, WO2006/027893A1 discloses a sensor for detecting a substance in liquid using a surface acoustic wave element. FIG. 5A is a plan view for describing a sensor for detecting a substance in liquid described in WO2006/027893A1, and FIG. 5B is a front cross-sectional view that illustrates a main portion thereof.
A sensor 101 for detecting a substance in liquid includes a base substrate 102. The base substrate 102 includes an upper surface 102a in which recesses 102c and 102d are provided at locations spaced from a first end 102b. A sensing SAW element 104 and a reference SAW element 105 are disposed in the recesses 102c and 102d, respectively. A resin layer 103 having holes 103b and 103c facing the recesses 102c and 102d, respectively, is laminated on the base substrate 102.
The sensing SAW element 104 includes a piezoelectric substrate, an interdigital transducer (IDT) provided on the piezoelectric substrate, and a reaction film arranged so as to cover the IDT. The reaction film is made of a material that reacts with a detection-target substance in liquid and is coupled to the detection-target substance. The reference SAW element 105 includes an IDT that is provided on a piezoelectric substrate. The reference SAW element 105 does not include a reaction film.
In use, at least the portions in which the holes 103b and 103c are disposed are placed in liquid, and as a result, the reaction film of the sensing SAW element 104 reacts with and couples to a detection-target substance. Accordingly, in the sensing SAW element 104, a mass on the portion in which the IDT is disposed is increased by the coupling to the detection-target substance. In contrast, in the reference SAW element 105, because the reaction film which reacts with the detection-target substance is not provided, there is no increase of mass caused by the coupling to the detection-target substance.
In the sensor 101 for detecting a substance in liquid described in WO2006/027893A1, a change in the speed of sound of a surface acoustic wave caused by an addition of mass in the sensing SAW element 104 is detected as a change in an electrical signal. In this case, the detection-target substance can be detected with relatively high precision by determining the difference between an output from the sensing SAW element and an output from the reference SAW element.
A similar sensor for detecting a substance in liquid is also disclosed in WO2006/027945A1.
As described above, the sensor 101 for detecting a substance in liquid detects the presence or absence and the density of a protein in liquid using the difference between an output signal from the sensing SAW element 104 and that from the reference SAW element 105. Specifically, the difference between an oscillation frequency of an oscillation circuit including the sensing SAW element and that of an oscillation circuit including the reference SAW element is determined in order to detect the presence or absence or the density of the detection-target substance.
In the sensor 101 for detecting a substance in liquid, it is highly desirable that, when the detection-target substance is not present, the characteristic of the sensing SAW element 104 and that of the reference SAW element 105 be substantially the same or approximately equal.
However, when the detection-target substance comes into contact with the SAW elements 104 and 105, the difference between the oscillation frequency of the oscillation circuit including the sensing SAW element 104 and that including the reference SAW element 105 may be relatively small. In such a case, both oscillations may be electromagnetically coupled, and both of the oscillation frequencies may be substantially or exactly the same. For this reason, the difference in frequency between the oscillation circuit including the reference SAW element and the oscillation circuit including the sensing SAW element is approximately zero, such that it is difficult to reliably detect the detection-target substance in liquid with high precision.